The present invention relates to a fuel supply control system for an internal combustion engine.
More particularly, the present invention pertains to the adjustment of the air-fuel mixing ratio of a combustible mixture to be supplied to the engine according to engine operating condition between values lower and higher, respectively, than the air-fuel mixing ratio at which the emission of nitric oxides (NO.sub.x) contained in exhaust gases emerging from the engine as a result of the combustion mixture having burned in the engine attains a maximum value.
As one example of prior art fuel supply control systems for automobile engines, Japanese Laid-open Patent Publication No. 59-96452, published June 2, 1984, discloses a system wherein, while a feedback or closed-loop control is employed as a rule to control the combustible mixture to be supplied to the engine to a stoichiometric air-fuel mixing ratio in dependence on the concentration of a selected constituent of the exhaust gases emitted as a result of the combustion of the combustible mixture which has taken place in the engine, the combustible mixture can be controlled to a higher air-fuel mixing ratio than the stoichiometric value during a particular engine operating condition.
Another example of a fuel supply control system is known wherein, while the combustible mixture is, as a rule, leaned as much as possible within a tolerance of roughness, the combustible mixture can be controlled to a lower air-fuel mixing ratio during acceleration or a high load engine operating condition to permit the engine to produce an increased type.
While the adjustment of the air-fuel mixing ratio, or briefly speaking, A/F ratio, based on the feedback control scheme has now been widely practiced in most automobile engine systems, the adjustment of the A/F ratio from a high value to a low value or from a lower value to a high value according to an engine operating condition is, in order to minimize the emission of nitric oxides (NO.sub.x), preferred to be carried out in such a way that the A/F ratio after and before such adjustment will not take a value at which the NO.sub.x emission is considered maximum and which is generally considered to be about 16. In other words, in order to minimize the NO.sub.x emission, the A/F ratio of the combustible mixture to be supplied to the engine according to an engine operating condition is preferred to be quickly adjusted between values lower and higher, respectively, than the A/F ratio at which the maximum NO.sub.x emission takes place. (It is to be noted that the A/F ratio at which the maximum NO.sub.x emission takes place, that is, the NO.sub.x emission attains a maximum value will be hereinafter referred to as "critical A/F ratio".) Where the A/F ratio adjustment is progressively carried out where the combustible mixture is desired to be adjusted from the lower A/F ratio to the higher A/F ratio or from the higher A/F ratio to the lower A/F ratio with respect to the critical A/F ratio to suit the requirement for a change in engine operating condition, the supply of the combustible mixture of critical A/F ratio to the engine is sustained for a substantial length of time and the NO.sub.x emission increases correspondingly if not attaining the maximum.
On the other hand, if the A/F ratio adjustment referred to above is quickly carried out, a torque shock is likely to occur in the engine as a result of abrupt change in A/F ratio of the combustible mixture being applied to the engine.